Firstly, Im not an expert, but Ive read that it has something to do with to much going on in the game at once. i.e. players running around with grenade launchers or ray gun with double tap, spraying. Or Ive had it happen when running at increased time scale and everyone began spraying guns.

The reason this happens is there are no free spawn points available. When you delete zombies it leaves remains of them in the spawn points clogging them up and eventually new zombies have nowhere to spawn. To fix this play on a map where the zombies walk out of hallways such as kino or 5 and never leave the spawn area and dont open doors

Well I was kind of half right. I think.